Indoor turbo sessions over the course of the last few years have had a complete makeover. At one time it was standard procedure to wear out bike tires and withstand hours in a fixed position. With the addition of smart trainers to the market, cyclists and triathletes alike are moving many of their outdoor sessions indoors. Indoor training brings consistency and ease for completing workouts exactly as prescribed, especially with the addition of ERG mode. 

Smart trainers, such as the KICKR, come equipped with two modes: level and ERG mode. Level mode is when you, the rider, control the resistance of the trainer and shift freely in order to produce your desired wattage. ERG mode is entirely different. In ERG mode, the trainer will automatically adjust the resistance in order to hit the pre-selected wattage target in a workout. You can think of it like a treadmill. You set a pace for the belt and you hold onto that pace for as long as you can. Whether you become fatigued or not the belt still moves at the prescribed pace. ERG mode does the same. You set the wattage, for example, 200 watts, and whether you pedal at 95 RPM or 65 RPM, the trainer will adjust resistance to ensure each pedal stroke is putting out 200 watts. Here comes the big question: does it matter what gear you are in when using ERG Mode?

First, let’s take a look at the design of the smart trainers to begin to understand why gear selection would make a difference. Smart trainers are designed to apply a specific resistance to the flywheel based on the speed and pre-selected power target. The smart trainer assumes that the variable to change is the resistance, assuming the flywheel speed remains constant. This can present a problem then when you try to change your cadence during an effort (either increase or decrease) when a higher power target comes on. There is a period of time, approximately 10 seconds, that your smart trainer will have to adjust the resistance of the flywheel because it was measuring resistance off of the previous speed. When you increase or decrease the speed of the flywheel, the trainer will have to adjust the resistance. In order to make the shift smoother, it is best to shift a few gears in order to create the least amount of impact change to the flywheel speed.

The inertia stored within the flywheel of your trainer can simulate different outdoor riding terrains. When riding indoors on a smart trainer there are some aspects to consider relative to each individual’s personal strengths and weaknesses when considering your gear choice in ERG mode.

If you are a time trialist or sprinter who typically puts out most of your outdoor power in a big gear, then we would suggest replicating efforts indoors in ERG mode in a similar gear choice when performing sprint and time trial efforts.

If you are trying to replicate climbing workouts on your smart trainer or prefer to ride in the small chainring, then the same idea applies; do workouts in ERG mode in smaller gears to replicate the same outdoor “feeling.” ERG mode allows riders to do workouts that replicate big gear, low cadence work. However, throwing your chain into 53×11 will generate a high speed within the flywheel that will not produce the same inertial load you were looking for if you were outdoors climbing.

The opposite can also be true if you try to perform high cadence work in ERG mode in a small gear. The inertia of the flywheel will be reduced and will feel as if you’re trying to spin up a climb rather than on the flats.

Whether working in a big or small gear, it is important to think about how your trainer is setting resistance to generate a power target based on the speed of the flywheel. For example, going from a speed of 10 mph to 15 mph is a larger jump in speed than 25 to 30 mph from a resistance standpoint. If you have ever used ERG mode, then you may have experienced the “Spiral of Death,” where you find yourself in a small gear, and when the power increases, your cadence drops thus increasing the resistance and you find yourself grinding to a complete halt.

If using ERG mode for workouts try the following: keep your gearing in mid cassette range most of the time. Try shifting up or down two gears when performing workouts with significant cadence variations. Low RPM efforts? High torque is needed, so be in a small gear. High cadence work? Be in the largest gear you can to build the flywheel momentum in order to spin it out! Still on the fence if gear selection matters? Try your own test at home and simulate target efforts at different RPMs to determine which gear selection replicates the same “outdoor” feel for you. What feels right for one rider will not be a blanket statement for all. The type of workout and target cadence for a workout will be the driving factor for gear choice when using ERG mode.



Mac Cassin is the Chief Cycling Physiologist at Wahoo Sports Science. He holds a degree in Integrative Physiology from the University of Colorado-Boulder and has won multiple National Championships. The experience of juggling athletic goals with collegiate and career responsibilities has taught Mac that peak performance is achievable even for those who cannot focus exclusively on training.  While concentrating on exercise physiology in an academic setting, Mac competed at the World Championships, Pan American Championships, and World Cups on both the road and track.

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